Screening of environmental friendly ionic liquid as a solvent for the different types of separations problem: Insight from activity coefficients at infinite dilution measurement using (gas + liquid) chromatography technique

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Highlights

  • Activity coefficients at infinite dilution measured in the ionic liquid [EMIM]+[OS].

  • 29 Organic solutes and water investigated at T = (318.15, 333.15, 348.15 and 358.15) K using GLC.

  • Selectivities for selected separations compared to other IL’s and solvents.

  • The expression and estimation of uncertainty have been described in detail.

Abstract

The present work focussed on screening of the environmental friendly imidazolium-based ionic liquid for the separations of (alkane/aromatic), (alkane/alk-1-ene), (cycloalkane/aromatic) and (water/alkan-1-ol) using (gas + liquid) chromatography (GLC) technique. In this reason the activity coefficients at infinite dilution, γ13, for 29 organic solutes (alkanes, cycloalkanes, alkenes, alkynes, aromatics, alkanols and ketones) and water in ionic liquid 1-ethyl-3-methylimidazolium n-octylsulphate [EMIM]+[OS] were measured at temperatures of (318.15, 333.15, 348.15 and 358.15) K. Stationary phase loadings of (23.77 and 40.76)% by mass were used to ensure repeatability of measurements. Uncertainties were determined using the law of propagation of uncertainty with an average absolute deviation of 2.53%. Density, viscosity and refractive index values were measured to confirm the purity of pure ionic liquid. Partial molar excess enthalpies at infinite dilution, ΔH1E,, were also determined. The selectivities, Sij, and capacities, kj, were determined for the separations of (alkane/aromatic), (alkane/alk-1-ene), (cycloalkane/aromatic) and (water/alkan-1-ol). The separating ability of the investigated ionic liquid was compared against previously investigated and industrial solvents such as sulfolane, n-methyl-2-pyrrolidine (NMP) and n-formylmorpholine (NFM).

Introduction

The need for accurate thermodynamic data is becoming increasingly important to the industrial applications. Many principles of separation and transport processes hinge on the basis of such measurements [1], most commonly used on processes such as distillation and solvent extraction (also called (liquid + liquid) extraction) [2].

Organic solvents are used in different industrial chemical processes with satisfying results [3]. However, most of these chemicals represent a risk to human health as well as the environments due to their volatile nature. Ionic liquids (ILs) have been recognised as a new class of green solvents which has brought much interest to both the industrial and academic domains [4], [5], [6]. ILs has unique properties such as wide liquid ranges, stability at high temperatures, no flammability and negligible vapour pressures [7], [8], [9], [10]. The potential applications of ILs with imidazolium cations have been studied by Stepnowski [11]. There are, however, disadvantages to the use of ILs with the foremost being the high cost as compared to currently used solvents [12].

Numerous investigations have been done on imidazolium-based ILs and much information is available in the open literature [3], [13], [14], [15], [16], [17]. However, there is a very limited amount of information for long chained sulphate containing ILs, specifically the n-octylsulphate [OS] anion. The IL [EMIM]+[OS] was selected as it has been shown that alkyl sulphate-based ILs are less expensive, more hydrolytically stable and more environmentally friendly than other ILs [4].

In this work, we explore the interactions of organic compounds and water with the IL 1-ethyl-3-methylimidazolium n-octylsulphate [EMIM]+[OS] by systematic (gas + liquid) chromatography (GLC) retention time measurements. The interactions of 29 organic solutes and water were determined with selected IL. The influence of the anion on the solvating ability of 1-ethyl-3-methylimidazolium based ILs has also been determined. This work is a continuation of our research group’s investigation of activity coefficients at infinite dilution of organic solutes in ILs [18], [19], [20], [21], [22] The 3D structures of the studied IL 1-ethyl-3-methylimidazolium n-octylsulphate [EMIM]+[OS] are shown in figure 1.

Section snippets

Materials

The IL, [EMIM]+[OS], was obtained from Merck with a purity of 0.980 by mole fraction. The IL was kept in a 50 mL round bottom flask and connected by means of an adapter and valve to a two stage Edwards’s vacuum pump. The flask was submerged in an ultra-sonic bath of silicon oil at T = 333.15 K for approximately 72 h, with vibrations lasting for the initial 3 h only. The resulting moisture content was measured as 23 ppm by Karl–Fischer titration. The solutes used are listed in table 1. No further

Results and discussion

The measurements of γ13 for solutes in the [EMIM]+[OS] are reported in table 3, with an average uncertainty of 2.53%. FIGURE 2, FIGURE 3, FIGURE 4, FIGURE 5, FIGURE 6, FIGURE 7, FIGURE 8, FIGURE 9, show the natural logarithm of the γ13 values as a function of the inverse absolute temperature for all solutes. The γ13 values were in good agreement with previously studied imidazolium based ILs, more so for investigations with the cation [EMIM]+. Studies by Bahadur et al. [3], Tumba et al. [18]

Conclusions

Using (gas + liquid) chromatography technique, the activity coefficients at infinite dilution were measured for 30 different solutes in [EMIM]+[OS]. The solvating ability was compared to other ILs containing the [EMIM]+ cation and industrial solvents. The investigated IL had displayed significantly lower selectivities for the various hydrocarbon separations chosen but higher capacities. No comparison for the water/butan-1-ol separation was possible at this stage. From the results, it seems

Acknowledgements

The authors acknowledge University of KwaZulu-Natal for undergraduate scholarships for M. Naidoo, S. Ramdath. This work is based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation. Dr I. Bahadur thanks to NRF/DST South Africa.

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